the .iso is install-powerpc-minimal-2008.0.iso
At the step "Copying the files"
Just do
# cp -r /mnt/cdrom/* /mnt/usb

Skip the Adjusting the bootloader configuration step

Boot on it (on mac, use Alt (Option) key to display bootable devices), it should boot into yaboot, then hit "Tab" to select the goof kernel (ppc) and that's it !

BUT : I only managed to boot it from one of the three ppc machines I have, sooooo maybe the Adjusting the bootloader configuration step was necessary, I don't know.
But it works, since I've managed to get a live cd prompt !
Comments are welcome _________________Marre de la modération ? Envie de flooder sans limites ? => Flood.fr est fait pour vous !